The present invention relates to a cryogenic refrigerator for cooling superconducting devices, and more particularly to a single stage, twin piston refrigerator for prolonged, low power cooling.
With the advent of superconducting quantum interference devices, a number of electronic instruments have been developed which are vastly superior to their conventional components in terms of sensitivity, operating speed and portability. These devices require a low temperature environment below 9.degree. to 10.degree. K. and preferably 3.degree. to 7.degree. K. for optimum performance. This low temperature environment is attained by the use of helium cryocoolers.
Prior art cryogenic refrigerators include the four stage cascade displacer and cylinder of Zimmerman, U.S. Pat. No. 4,143,520, which can attain a temperature on the order of 8.5.degree. to 13.degree. K. The cascaded nylon displacer of Zimmerman is annealed into precise conformation with the epoxy-glass composite cylinder to form optimum clearances upon cooling and thus enabling the low temperatures to be achieved.
The main limitation on the cold end temperature of this machine is the heat regenerative capacity. The total regenerator heat capacity of the final stage, compared to that of the working fluid, is too small to provide efficient regenerative heat exchange.
To obtain temperatures lower than that permitted by the Zimmerman cryogenic refrigerator, it is necessary to provide more efficient regenerative heat exchange between the displacer and the working fluid.